Pinned tabs are not reopened in same container

[bomgar]

Pinned tabs are not reopened in same container

[dannycolin]

The addon opens a new tab on firefox relaunch and asks me what to do.

This is the expected behavior. To solve this issue, you have two choices:

a) tick the "Remember my decision" box so it's permanently assigned to a container
b) remove the assignment for this website.

[bomgar]

that might be true for normal tabs. It is strange for pinned tabs though.
The tab that asks you which container you want is also not pinned anymore.
so not really expected.

[dannycolin]

It is strange for pinned tabs though.

Why is it strange. Pinned tabs aren't tied up to the container mechanism and exist on their own. They exist primarily to avoid a tab to be closed.

The tab that asks you which container you want is also not pinned anymore.

If your pinned tabs aren't pinned anymore when you reopen your browser, this is most likely a bug with Firefox itself. You can report it on bugzilla.mozilla.org.